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+====== An example of custom protocol for Modbus ASCII ======
+  ***Type**: Serial port
+  ***Address validation**: ^(C[0-9]+)$|^(DI[0-9]+)$|^(HR[0-9]+)$|^(IR[0-9]+)$
+  ***Validation error message**: Invalid register address. Valid ModBus addresses are Cxxx, DIxxx, IRxxx, HRxxx.
+Code:
+<code lua>
+-- MODBUS ASCII Demo Driver
+function parseAddress ()
+  -- do something here
+end
+function createDevices ()
+  addDevice({name = "C",  shift = 0, base = 10, xtraFields = {1, 5}});
+  addDevice({name = "DI", shift = 0, base = 10, xtraFields = {2, 0}});
+  addDevice({name = "HR", shift = 0, base = 10, xtraFields = {3, 6, 16}});
+  addDevice({name = "IR", shift = 0, base = 10, xtraFields = {4, 0}});
+end
+local errorCount = 0;
+function intToPair(val, count)
+    local str = string.upper(string.format("%0" .. count .. "x", val));
+    local res = {};
+    for i = 1, count do
+        res[i] = string.byte(string.sub(str, i, i));
+    end
+    return res;
+end
+function pairToInt(a, b)
+    local str = string.char(a) .. string.char(b);
+    local n = tonumber(str, 16);
+    return n;
+end
+function LRC(table, start, count)
+    local lrc = 0;
+    local i = start;
+    while (i < (start+count)) do
+        local str = string.char(table[i]) .. string.char(table[i + 1]);
+        local n = tonumber(str, 16);
+        lrc = lrc + n;
+        i = i + 2;
+    end
+    lrc = bit.band(lrc, 255);
+    return 256 - lrc;
+end
+function readRegister (reg, device, unitId)
+  local request = {};
+  -- ASCII header
+  request[1] = 58;
+  -- slave address
+  local addr = intToPair(unitId, 2);
+  request[2] = addr[1];
+  request[3] = addr[2];
+  -- function code
+  local fnCode = intToPair(device.xtraFields[1], 2);
+  request[4] = fnCode[1];
+  request[5] = fnCode[2];
+  -- address of register
+  local intAddr = intToPair(reg.internalAddr, 4);
+  request[6] = intAddr[1];
+  request[7] = intAddr[2];
+  request[8] = intAddr[3];
+  request[9] = intAddr[4];
+  -- count of registers
+  local count = 1;
+  if (reg.dataType == 3) then -- double word
+    count = 2;
+  end
+  count = intToPair(count, 4);
+  request[10] = count[1];
+  request[11] = count[2];
+  request[12] = count[3];
+  request[13] = count[4];
+  -- LRC
+  local lrc = LRC(request, 2, 12);
+  lrc = intToPair(lrc, 2);
+  request[14] = lrc[1];
+  request[15] = lrc[2];
+  -- CR LF
+  request[16] = 13;
+  request[17] = 10;
+  local res = sendBytes(request);
+  if (res == false) then
+      DEBUG("Can't send bytes");
+      return false;
+  end
+  local response = {};
+  -- read Header
+  response = readBytes(7);
+  if (response == false) then
+      DEBUG("Can't read response");
+      return false;
+  end
+  res = #response;
+  if (res ~= 7) then
+      errorCount = errorCount + 1;
+      if (errorCount > 3) then
+          closeConnection();
+          errorCount = 0;
+      end
+      DEBUG("Can't read header");
+      return false;
+  end
+  if (response[1] ~= request[1]) then
+      ERROR("Wrong protocol");
+      return false;
+  end
+  if (response[2] ~= request[2] or response[3] ~= request[3]) then
+      ERROR("Wrong UnitID in response");
+      return false;
+  end
+  if (response[4] ~= request[4] or response[5] ~= request[5]) then
+      ERROR("Wrong function code in response");
+      return false;
+  end
+  local length = pairToInt(response[6], response[7]);
+  if (length < 1) then
+      ERROR("Wrong length in response");
+      return false;
+  end
+  local response2 = {};
+  response2 = readBytes(length * 2 + 4);
+  if (response2 == false) then
+      DEBUG("Can't read response2");
+      return false;
+  end
+  res = #response2;
+  local result = {};
+  local resp = {};
+  for i=1,#response do
+      resp[i] = response[i];
+  end
+  local c = #response;
+  for i=1,#response2 do
+      resp[c+i] = response2[i];
+  end
+  local lrc_resp = LRC(resp, 2, 6 + length * 2);
+  local lrc_actual = pairToInt(response2[length*2 + 1], response2[length*2 + 2]);
+  if (lrc_resp ~= lrc_actual) then
+      ERROR("Wrong LRC in response");
+      return false;
+  end
+  if (length >= 1) then
+      for i = 1, length do
+          result[i] = pairToInt(response2[i*2 - 1], response2[i*2]);
+      end
+  end
+  return result;
+end
+function writeRegister (reg, device, unitId, newValue)
+    local request = {};
+    -- ASCII header
+    request[1] = 58;
+    -- slave address
+    local addr = intToPair(unitId, 2);
+    request[2] = addr[1];
+    request[3] = addr[2];
+    if (reg.dataType == 3) then -- double word should be handled in a special way
+        if (device.xtraFields[3] == 0) then
+            ERROR("Can't write these type of registers (" .. device.name .. ")");
+            return false;
+        end
+        -- function code
+        local fnCode = intToPair(device.xtraFields[3], 2);
+        request[4] = fnCode[1];
+        request[5] = fnCode[2];
+        -- address of register
+        local intAddr = intToPair(reg.internalAddr, 4);
+        request[6] = intAddr[1];
+        request[7] = intAddr[2];
+        request[8] = intAddr[3];
+        request[9] = intAddr[4];
+        local count = intToPair(2, 4);
+        request[10] = count[1];
+        request[11] = count[2];
+        request[12] = count[3];
+        request[13] = count[4];
+        -- message length
+        local dataLen = intToPair(4, 2);
+        request[14] = dataLen[1];
+        request[15] = dataLen[2];
+        local valpairs = intToPair(newValue, 8);
+        for i=1,8 do
+            request[15+i] = valpairs[i];
+        end
+        -- LRC
+        local lrc = LRC(request, 2, 22);
+        lrc = intToPair(lrc, 2);
+        request[24] = lrc[1];
+        request[25] = lrc[2];
+        -- CR LF
+        request[26] = 13;
+        request[27] = 10;
+        local res = sendBytes(request);
+        if (res == false) then
+            DEBUG("Can't send request");
+            return false;
+        end
+        local response = {};
+        response = readBytes(17);
+        if (response == false) then
+            DEBUG("Can't read response");
+            return false;
+        end
+        res = #response;
+        if (res ~= 17) then
+          DEBUG("Wrong response length");
+          return false;
+        end
+        if (response[1] ~= request[1]) then
+          ERROR("Wrong protocol");
+          return false;
+        end
+        local lrc_resp = LRC(response, 2, 12);
+        local lrc_actual = pairToInt(response[14], response[15]);
+        if (lrc_resp ~= lrc_actual) then
+          ERROR("Wrong LRC in response");
+          return false;
+        end
+        if (response[2] ~= request[2] or response[3] ~= request[3]) then
+          ERROR("Wrong UnitID in response");
+          return false;
+        end
+        if (response[4] ~= request[4] or response[5] ~= request[5]) then
+          ERROR("Wrong function in response");
+          return false;
+        end
+        if (response[6] ~= request[6] or response[7] ~= request[7] or response[8] ~= request[8] or response[9] ~= request[9]) then
+          ERROR("Wrong register address in response");
+          return false;
+        end
+        if (response[10] ~= request[10] or response[11] ~= request[11] or response[12] ~= request[12] or response[13] ~= request[13]) then
+          ERROR("Wrong register count");
+          return false;
+        end
+        if (response[16] ~= request[16] or response[17] ~= request[17]) then
+          ERROR("Wrong footer in response");
+          return false;
+        end
+    else
+        if (device.xtraFields[2] == 0) then
+            ERROR("Can't write these type of registers (" .. device.name .. ")");
+            return false;
+        end
+        -- function code
+        local fnCode = intToPair(device.xtraFields[2], 2);
+        request[4] = fnCode[1];
+        request[5] = fnCode[2];
+        -- address of register
+        local intAddr = intToPair(reg.internalAddr, 4);
+        request[6] = intAddr[1];
+        request[7] = intAddr[2];
+        request[8] = intAddr[3];
+        request[9] = intAddr[4];
+        -- data
+        local val = newValue;
+        if (reg.dataType == 0) then
+            if (val > 0) then
+                val = 255*256;
+            else
+                val = 0;
+            end
+        end
+        local valpairs = intToPair(val, 4);
+        request[10] = valpairs[1];
+        request[11] = valpairs[2];
+        request[12] = valpairs[3];
+        request[13] = valpairs[4];
+        -- LRC
+        local lrc = LRC(request, 2, 12);
+        lrc = intToPair(lrc, 2);
+        request[14] = lrc[1];
+        request[15] = lrc[2];
+        -- CR LF
+        request[16] = 13;
+        request[17] = 10;
+        local res = sendBytes(request);
+        if (res == false) then
+            DEBUG("Can't send bytes");
+            return false;
+        end
+        local response = {};
+        local requestLen = #request;
+        response = readBytes(requestLen);
+        if (response == false) then
+            DEBUG("Can't read response");
+            return false;
+        end
+        res = #response;
+        if (res ~= requestLen) then
+          DEBUG("Wrong response length");
+          return false;
+        end
+        for i = 1,res do
+            if (response[i] ~= request[i]) then
+                DEBUG("Wrong response");
+                return false;
+            end
+        end
+    end
+    return true;
+end
+</code>